This application claims the priority of Japanese Patent Application No. 10-375978/1998 filed on Dec. 18, 1998 and No. 11-301823/1999 filed on Oct. 25, 1999, which are incorporated herein by reference.
The present invention relates to a lithium secondary battery comprising a positive electrode using, as an active material, a lithium-containing manganese composite oxide having a spinel structure, and the positive electrode used therein. More particularly, it relates to improvement of an active material of a positive electrode for the purpose of providing a high-voltage lithium secondary battery exhibiting good charge-discharge cycle performance.
As positive electrode materials (positive electrode active materials) for lithium secondary batteries, LiCoO2 and LiNiO2 are conventionally well known, but these positive electrode materials are expensive and hence disadvantageous in material cost.
Therefore, lithium-containing manganese oxides such as a lithium-containing manganese oxide having a spinel structure (LiMn2O4) and an orthorhombic lithium-containing manganese oxide (LiMnO2) have been proposed as the positive electrode active material. Such a lithium-containing manganese oxide is one of promising materials for a positive electrode of a lithium secondary battery because manganese, the raw material, is an abundant resource and hence inexpensive.
However, when the conventional lithium-containing manganese oxide is used, the discharge potential versus lithium (Li/Li+) is as low as 4.2 V or less and hence the discharge capacity is small.
As a positive electrode material having a potential plateau in the vicinity of 4.7 V and a discharge potential of 4.5 V or more versus lithium, a lithium-containing manganese oxide represented by a formula, Lix-yMzMn2-y-zO4 (wherein 0xe2x89xa6x less than 1, 0xe2x89xa6y less than 0.33 and 0 less than z less than 1), has been proposed (in U.S. Pat. No. 5631104 and Japanese Laid-Open Patent Publication No. 9-147867/1997). This lithium-containing manganese oxide is obtained by substituting another transition metal M, such as nickel and chromium, for part of manganese of a lithium-containing manganese oxide having a spinel structure.
This lithium-containing manganese oxide, however, was found to have a problem of poor charge-discharge cycle performance because its crystal structure is partially changed through repeated charge and discharge.
The present invention was devised in view of these conventional problems, and the object is providing a high-voltage lithium secondary battery exhibiting good charge-discharge cycle performance. This object is achieved by utilizing a specific lithium-containing manganese oxide as an active material of a positive electrode as described in detail below.
In order to achieve the aforementioned object, the lithium secondary battery of this invention (present battery) comprises a positive electrode using, as an active material, a lithium-containing manganese composite oxide with a spinel structure having a composition, during charge and discharge, represented by a formula, LixMn2-y-zNiyMzO4, wherein M is at least one element selected from the group consisting of Fe, Co, Ti, V, Mg, Zn, Ga, Nb, Mo and Cu; 0.02xe2x89xa6xxe2x89xa61.10 (whereas x changes in accordance with occlusion and discharge of lithium ions during charge and discharge); 0.25xe2x89xa6yxe2x89xa60.60; and 0 less than zxe2x89xa60.10. Specifically, the positive electrode active material of the present battery is a lithium-containing composite oxide in which Ni and a specific element M such as Fe are substituted for part of Mn. Since Ni is thus substituted for part of Mn, the positive electrode active material attains a stable crystal structure even at a high potential of 4.5 V or more. Also, since the specific element M such as Fe is substituted for part of Mn, the ordering energy can be lowered, resulting in reducing strain of the crystal structure caused during repeated charge and discharge. Accordingly, the present battery can exhibit good charge-discharge cycle performance.